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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Hard error when checking validity of protected default constructor in SFINAE context"
href="https://bugs.llvm.org/show_bug.cgi?id=43454">43454</a>
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<th>Summary</th>
<td>Hard error when checking validity of protected default constructor in SFINAE context
</td>
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<th>Product</th>
<td>clang
</td>
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<th>Version</th>
<td>trunk
</td>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>All
</td>
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<th>Status</th>
<td>NEW
</td>
</tr>
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<th>Severity</th>
<td>enhancement
</td>
</tr>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>Frontend
</td>
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
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</tr>
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<th>Reporter</th>
<td>ldionne@apple.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org, neeilans@live.com, richard-llvm@metafoo.co.uk
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<pre>The following code causes a hard compilation error on Clang trunk in all
standard modes, except C++17:
#include <utility>
#include <type_traits>
struct Base { };
struct Derived : Base { protected: Derived() = default; };
template <class _Tp>
void test_implicit_default_constructible(_Tp);
template <class _Tp, class = void>
struct is_implicitly_default_constructible
: std::false_type
{ };
template <class _Tp>
struct is_implicitly_default_constructible<_Tp,
decltype(test_implicit_default_constructible<_Tp const&>({}))>
: std::is_default_constructible<_Tp>
{ };
static_assert(!is_implicitly_default_constructible<Derived>::value, "");
You can reproduce with:
$ cat <file above> | clang++ -xc++ - -std=c++2a -fsyntax-only
<stdin>:17:110: error: calling a protected constructor of class 'Derived'
struct is_implicitly_default_constructible<_Tp,
decltype(test_implicit_default_constructible<_Tp const&>({}))>
^
<stdin>:20:20: note: in instantiation of template class
'is_implicitly_default_constructible<Derived, void>' requested
here
static_assert(!is_implicitly_default_constructible<Derived>::value,
"");
^
<stdin>:5:40: note: declared protected here
struct Derived : Base { protected: Derived() = default; };
^
1 error generated.
Live repro: <a href="https://godbolt.org/z/e9-G8b">https://godbolt.org/z/e9-G8b</a>
Instead, I would expect the static_assert to pass since Derived isn't default
constructible (because of access checking), and we check that inside a SFINAE
context (so no hard error should happen).
Note that Clang produces an error in C++14 and C++2a modes, but produces no
error in C++17 mode. GCC trunk, in comparison, produces no error in C++14,
C++17 and C++2a mode.</pre>
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